Oliver
since you bought the parts from us,
call me later (after 8.30 Pacific)
I can guide you
on how to and make sure you done it right

on the group head piston, there are two groves - for two gaskets.

The top gasket should flare out at the top

The bottom gasket should flare out at the bottom.

In other words, when you move the piston up, the upper gasket should seal out against the cylinder wall and push any air/water/steam back towards the boiler. In fact, when you raise the lever, it normally should produce a slight increase in pressure in the boiler (for a split second), which will be visible on the pressure gauge (if so equipped).

The bottom gasket should produce a seal on the downward stroke, as the lower gasket flares out against the cylinder wall - i.e. during extraction.

When you install the seals into the group head cylinder, it's usually wise to coat them with a very, very slight amount of vegetable or olive oil. It's also important to ensure the upper seal / lip doesn't get caught or pinched during installation, as this can tear or cut the upper gasket.

Ray